Gripper pad

ABSTRACT

A gripper pad, for use in a track-type continuous rod service rig, is provided. The pad comprises a deformable elastomer block having a series of aluminum studs embedded in its upper face. The provision of the studs results in higher pressure being applied to the rod, against which the pad is squeezing, than is applied to the block itself by the rig track.

0 q ilmte States Patent 11 1 1111 3,754,474 Palynchuk 1 Aug. 28, 1973GRIPPER PAD 3,305,048 2/1967 Brilando 188/251 3,585,102 6/1971 Bur ess188/251 [751 Invent: Alexander Palynchuk, Edmnmn, 2,775,869 1/1957 P015174/162 Alberta Canada 2,841,961 7/1958 Lucas 74/162 [73] Assignee: CorodManufacturing Ltd.,

Edmonton, Alberta, Canada Primary Examiner-Charles .l. Myhre AssistantExaminerWesley S. Ratliff, Jr. [22] Flled' sept' 1971 Attorney-ErnestPeter Johnson 21] App1.No.: 176,952

[57] ABSTRACT [52] US. Cl. 74/162, 188/251 A [51] Int. Cl. F16d 41/20 Agrfpper f for use m a track'type cfmtmuous rod [58] Field of Search.188/24 7 4/162, serv1ce rig, 1s provided. The pad compnses a deform-26o/75 77 5 able elastomer block having a series of aluminum studsembedded in its upper face. The provision of the studs [56] ReferencesCited results in higher pressure being applied to the rod, against whichthe pad is squeezing, than is applied to UNITED STATES PATENTS the blockitself by the rig track. 1,891,723 12/1932 Novak 188/251 3,003,60910/1961 Lang 4 Claims, 7 Drawing Figures PAIENIEDmzams 3,754,474

SHEET 2 OF 2 END LOAD I? S.

l I l l l l 0 I000 2000 3000 4000 5000 $00 GRIPPER PAD BACKGROUND OF THEINVENTION This invention relates to a gripper pad. More particularly, itis concerned with a gripper pad which is adapted for use in one of theendless tracks of a continuous string service rig.

In US. Pat. No. 3,559,905, I describe a service rig which is suitablefor tripping a continuous rod or tubing string (generically termed inthe claims as an oilfield string"). The rig is equipped with a pair ofopposed, spaced, endless tracks. Each track includes a sprocketdrivenchain which carries a row of gripper pads. In operation, the rod stringis gripped between the opposed rows of gripper pads and the tracks aredriven to feed the string into or out of the well.

SUMMARY OF THE INVENTION It is an object of the invention to provide anovel gripper pad which is sufficiently flexible to accomodate normalchanges in rod or tubing diameter and the passage of paraffin scrapersand yet which can be squeezed against the oily, wax-covered string togenerate the frican upper face which is termed its working face. Thisface is preferably concave in contour. The block itself is formed of anelastomer, preferably having a hardness greater than 80 on the Shore Ascale. In the best mode known to me, the block is formed of polyurethanehaving a hardness within the range 45 to 55 on the Shore D scale.

The studs are embedded in the block. The upper end surface of each studis disposed substantially flush with the blocks working face. The studis preferably made of a metal such as aluminum or aluminum alloy sincethese materials have desirable work-hardening characteristics and a highcoefficient of friction when in contact with steel. They are also softerthan steel and thus will not score the string.

The block is encased in a rigid container to provide a gripper unit".The container tightly encloses the ends, bottom and sides of the blockand, when in use, is affixed to the rig track.

It will be appreciated that a very large side force must be applied bythe track and gripper pad on the rod or tubing string to generatefrictional forces in an amount sufficient to suspend the string in awell. For example, in order to suspend a 6,000 foot long string of 5t;inch diameter oil-coated rod, it may be necessary to apply a side forceon the rod of 4000 p .s.i. or more. A side force of this magnitude tendsto deform the pad at it's working face to such an extent that shearingor failure of the elastomer can easily occur. By incorporating therigid, free-floating studs into the elastomer block, I find that higherside forces are generated at the rod face then would be the case if thestuds were not present.

This increment in force appears to arise from the movement of theelastomer past the stud body. The increase in side force, of course,means that there is a corresponding increase in friction force.Additionally, the high co-efficient of friction which derives from thecontact of metal with metal is an aid in increasing the friction forces.

The novel pad has two particularly advantageous features: it can deformto accomodate changes in size of the rod string moving past it; inaddition, it is adapted to provide adequate friction forces at the rodface without undue deformation of the elastomer.

DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a perspective view of the gripper unit;

FIG. 2 is a bottom view of the gripper pad;

FIG. 3 is an end view of the gripper pad;

FIG. 4 is a section view along the line AA in FIG.

FIG. 5 is a side view of a stud; FIG. 6 is a side view of another stud;FIG. 7 is a plot of the side load applied to a pair of opposed gripperpads versus the end load required to cause a rod gripped between thepads to slip. Curve 1 relates to gripper pads equipped with studs. Curve2 relates to gripper pads without studs.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to FIG. 1, a gripperunit is designated by the numeral 1. The gripper unit 1 consists of acontainer 2 and a gripperpad 3. The pad 3 includes a block 4 havingstuds 5 embedded therein. Collapse holes 6 are formed in the back of theblock 4.

The block 4 is generally rectangular in shape. It has a concave upperworking face 7 and one or more spaced, stud-receiving cavities 8 formedin its upper segment. These cavities 8 are aligned in spaced relationalong the longitudinal axis of the block 4. A plurality of downwardopening, inward directed, tapered holes 6 are formed in the lowersegment of the block 4.

The block 4 is formed of a defonnable elastomer material. It has ahardness greater than 80 on the Shore A scale; if it is softer, thematerial may deform to such an extent that failure can occur. I preferto use polyurethane having a hardness within the range 45 55 on theShore D scale. This material is capable of transmitting the loads whichare involved without much deformation; yet it is sufficiently flexibleto accomodate normal variations in string diameter. The best elastomerfor this use of which I am aware has a hardness within the preferredrange and has the following physical characteristics:

compression set (ASTM B) 33% tear strength (psi) I tensile strength(psi) 7500 elongation 400 The collapse holes 6 are provided in thebottom segment of the block 4. These holes 6 are not essential; however,when the block 4 is subjected to load, the elastomer deforms around andcloses the holes rather than bulging out at the working face 7, wherethe danger of failure of the material is greater. The size and shape ofthe holes 6 has some efi'ect on the performance of the elastomer. Testsshow that if the holes have a diameter greater than about 15/64 inches,the

proportional relationship between side load and deformation is lost;this is not desirable as the pad begins to act as a rigid block. Iprefer to provide holes which are inches deep and taper inwardly from adiameter of 13/64 inches to a diameter of 9/64 inches. The centers ofthe holes are spaced about it; inches apart.

The rigid studs 5 are pressed into the cavities 8. The top surface orend of each stud S is preferably flush with the block working face 7,although the surface may be located a small distance above or below theworking face 7 without affecting the operation of the gripping pad 3.The inner end of each stud 5 is spaced inwardly from the face of theblock 4 which is opposite to the working face 7. The studs 5 are formedof a metal which is softer than steel, preferably aluminum or analuminum alloy. They may be provided with locking means, such as thegroove 9, shown in FIG. 5, or the flange 10, shown in FIG. 6. Studswhich are of very small diameter, in the order of less than one-eighthinch, have a tendency to shift into a misaligned position when loaded;therefore, studs having a diameter of about one-half inch or larger arepreferable. When using a 4 X 1.25 X 1 inch gripper block, Iprefer topro:

vide 0.50 inch diameter x 057mm long cylindriail studs in combinationwith 0.90 inch square x 0.69 inch long square studs, as illustrated inFIG. 1.

In FIG. 8, curves 1 and 2 illustrate the surprising advantage which isgained by the use of the studs. The curves shown are developed bygripping an oily rod between two gripper pads using rams to squeeze thepads against the rod. Another ram is applied axially to the rod. Thepressure at which the rod slips is recorded for different side loads. Ata side load of 4000 p.s.i., it will be noted that there is a 100%improvement obtained as a result of including the studs in the gripperpad.

The gripper pad 3 fits into a groove 11 formed in container 2. The sidewalls of the container 2 extend up to the working face 7. Pins 12 areinserted through openings 13 to abut the ends of the pad. In thismanner. the pad 3 is restrained in all directions but one. The container3 is attached in any conventional manner to the tracks of the rod rig.

In operation, the tracks of the rig press two opposed gripper units 1against a rod string which extends between them. The elastomer block 4grips the rod firmly without biting into it and is capable oftransmitting side forces in the order of 4000 p.s.i. without failure.The rigid container and rod minimize deformation of the elastomer. Thestuds 5 bite through the oil film on the rod and transmit a side forceto it which is greater than the force originally applied to theelastomer block.

What is claimed is:

1. A gripper unit comprising:

a gripper pad comprising a deformable elastomer block having a workingface,

and

at least one rigid stud embedded in the block, the

outer end of the stud being disposed substantially flush with theworking face and its inner end being spaced inwardly from that face ofthe block which is opposite to the working face; and

rigid container means tightly encasing the sides of the block, up to theworking face, as well as the end and bottom surfaces of the block.

2. The gripper unit as set forth in claim 1 wherein:

the elastomer has a hardness greater than about shore A.

3. The gripper unit as set forth in claim 1 wherein:

the elastomer is polyurethane having a hardness within the range 45 55shore D, and the stud is formed of aluminum.

4. The gripper unit as set forth in claim 2 wherein:

the block has a plurality of small, inwardly extending holes formedtherein whereby the elastomer may initially be deformed to close theholes.

* It 1r

1. A gripper unit comprising: a gripper pad comprising a deformableelastomer block having a working face, and at least one rigid studembedded in the block, the outer end of the stud being disposedsubstantially flush with the working face and its inner end being spacedinwardly from that face of the block which is opposite to the workingface; and rigid container means tightly encasing the sides of the block,up to the working face, as well as the end and bottom surfaces of theblock.
 2. The gripper unit as set forth in claim 1 wherein: theelastomer has a hardness greater than about 80 shore A.
 3. The gripperunit as set forth in claim 1 wherein: the elastomer is polyurethanehaving a hardness within the range 45 - 55 shore D, and the stud isformed of aluminum.
 4. The gripper unit as set forth in claim 2 wherein:the block has a plurality of small, inwardly extending holes formedtherein whereby the elastomer may initially be deformed to close theholes.